Thanks all...but for some reason the discussion has veered off course a bit and that`s probably due in no small part to the experiential differential between the veteran DIY`selfers and those like myself who are looking to have a bit of the unnecessary tanglewood eliminated.
In an ideal world we`d all have the kopecks, experience and time to pursue penultimate designs and applications.
But when I`ve encountered similar questions posed by less inexperienced audio enthusiasts looking to maximize sonic enjoyment for minimum outlay I`m always interested in determining if their expectations exceed reason.
Clearly to some, what I`m proposing seems to be an affront to some rarefied sensibilities of what constitutes credible tube design on a limited budget.
But I sincerely doubt that the tube world is any different than the solid-state world where countless millions were duped into believing that only the most esoteric, expensive, high-powered and high-falutin manifestations of amplifier designs were worthy of consideration by serious audio enthusiasts.
If I had a dollar for every low-budget system properly synergized and balanced using proper speaker placement, room treatment and component matching that sounded just lovely when not over-extended beyond its capabilities...i`d have enough money to indulge in a no-holds-barred tube audio build.
I truly appreciate the input of everyone who`s taken the time to respond.
In an ideal world we`d all have the kopecks, experience and time to pursue penultimate designs and applications.
But when I`ve encountered similar questions posed by less inexperienced audio enthusiasts looking to maximize sonic enjoyment for minimum outlay I`m always interested in determining if their expectations exceed reason.
Clearly to some, what I`m proposing seems to be an affront to some rarefied sensibilities of what constitutes credible tube design on a limited budget.
But I sincerely doubt that the tube world is any different than the solid-state world where countless millions were duped into believing that only the most esoteric, expensive, high-powered and high-falutin manifestations of amplifier designs were worthy of consideration by serious audio enthusiasts.
If I had a dollar for every low-budget system properly synergized and balanced using proper speaker placement, room treatment and component matching that sounded just lovely when not over-extended beyond its capabilities...i`d have enough money to indulge in a no-holds-barred tube audio build.
I truly appreciate the input of everyone who`s taken the time to respond.
Jim Hagerman has a nice calculator for doing this, which is what I used for my 6GK5 phonostage. Good tube.
No, I think the verbosity of the posts has made it difficult to wade through. And, everyone likes a good phono stage discussion.
Anyhow, I posted a link to my phono stage above. I'd consider that a pretty simple build. For power amp + preamp, really what you need to figure out it gain. If you are using EL84 outputs, gain is not such a huge issue. So, how about a pot + cathode follower for a preamp, and a 6SN7 driving an EL84 for a power amp. Use simple CCS circuits for the loads (single 10M45 is fine), and you should be pretty good to go. Probably can't get any cheaper than that.
I gather you wish to keep it cheap by utilising the salvaged iron you already have. I have built a clone of the Decware Zen that worked out pretty well, but only a couple of watts. You might want to check out the tubelab site for the Simple SE Simple SE You can use 6V6 in place of the EL34 or KT88 if you want to use your salvaged transformers. I have built one of these and it is very good. If you are happy to purchase new output transformers, you might be interested in a PP design that utilises your EL84 tubes such as Eli Duttmans's El Cheapo http://homepage.mac.com/planet10/forum/elCheapo-23jun06-map.gif or Tubelab's Simple PP Simple P-P
If you really are interested in tube amps, an invaluable resource is teh books by Morgan Jones - 'Valve Amplifers' (design and theory) and 'Building Valve Amplifiers' (construction techniques). You probably know this already, but much of the performance with a tube amp is dependent on the somewhat black art of layout and construction. Poor layout and grounding technique will give mediocre results (noise and hum), regardless of the 'quality' of the components used.
If on a budget, it is worth noting that tube and capacitor purchases through ebay from the various Eastern Block sellers can be exceptional value. I have used them with good results and no issues from the various sellers such as SOVCOM items - Get great deals on HAM Radio, SN74... items on eBay Stores!
The big cost is the iron. If you have salvaged power transformers and output transformers, that is most of your cost right there. For info, Edcor are highly regarded on this forum for their good value for money power and output transformers EDCOR Electronics Corporation. Class X Transformers. Others have also had good results with Allied power transformers and Antek torroidal power transformers.
If you really are interested in tube amps, an invaluable resource is teh books by Morgan Jones - 'Valve Amplifers' (design and theory) and 'Building Valve Amplifiers' (construction techniques). You probably know this already, but much of the performance with a tube amp is dependent on the somewhat black art of layout and construction. Poor layout and grounding technique will give mediocre results (noise and hum), regardless of the 'quality' of the components used.
If on a budget, it is worth noting that tube and capacitor purchases through ebay from the various Eastern Block sellers can be exceptional value. I have used them with good results and no issues from the various sellers such as SOVCOM items - Get great deals on HAM Radio, SN74... items on eBay Stores!
The big cost is the iron. If you have salvaged power transformers and output transformers, that is most of your cost right there. For info, Edcor are highly regarded on this forum for their good value for money power and output transformers EDCOR Electronics Corporation. Class X Transformers. Others have also had good results with Allied power transformers and Antek torroidal power transformers.
I think this would be an excellent combination, while I did not mention it I am also a big fan of the 6J5 and 6SN7, both of which are used in my current 300B SE power amplifier design. The 6B4G (basically a 2A3 with a 6.3V filament) might be an interesting DHT choice for an output tube - it's on an octal base and I still see these relatively cheap from time to time at hamfests. (It has gotten a bit pricey on eBay, but it is worth a look there too) Less sensitive than the 6BQ5 or 6V6 in triode it can still be driven well by a 6SN7 with a CCS or choke load.
Thank-you kevinkr...as I started my premise out with the fact that I have significant stock of NOS 6GW8s and EL84s on hand there seems to be a lot of information regarding amplifier builds using these tubes.
While I'm certain these tubes have their obvious limitations, my on hand iron seems well-suited to pursuing a amplifier build based on these tubes in terms of economy and should I ultimately find my salvaged iron lacking could always be adapted to newer
With my on hand stock of 6sn7s and 7025s the Octal Hagerman Cornet Hybrid variations seem to be an avenue I could follow for a well-regarded MM phono pre.
And dsavitsk's advice regarding another phono stage avenue as well as integrating a pre stage within a power-amp build seems logical as well given my constraints.
Thks to all.
After a little more research I can see the Octal Hagerman Cornet variation is well outside my budget, the cost of the caps alone would exceed the GDP of a small country, methinks ..my inexperience reveals itself, yet again, so I'll stick with researching the well considered suggestions put forward thus far by much more experienced DIYAudio members.
It seems like the phono stage will be the hardest part to get right. As has been pointed out the main amp can be something as simple as an RH84 variant. As to the loading problems for the 12AX7/7025 (7025 would be the obvious choice of the two) you could use one of your 6SN7s on top and use a mu-follower stage to get a pretty good load on it. That doesn't solve the Cm problem (actually technically I suppose it would aggravate it due to the higher gain) but it might be possible to tweak the eq a bit to compensate.
Also it seems that the main problems with loading on the 12AX7 would be the effect on the eq circuit that it drives and the distortion that results from less than optimum load. The former can be dealt with by taking the rp into account in the calculation of the eq circuit values (simulation with LTspice could be useful here).
Mitigating the distortion problem is also the fact that the voltage swing will be comparatively small which will limit the effect of non-linearities. By choosing the the values used in the network you may be able to come up with something that though far from ideal might be quite listenable. Of course the mu-follower (or even putting a CF between the AX7 and the eq network) would pretty well solve the distortion issue. It is getting a little more complex but the extra tubes are not necessarily expensive (even 12AU7 can be used for follower or plate load).
Another thought is that the 12AY7 is not super expensive and would provide less of a Cm problem. I looked at the GE data sheets for all three tubes and the 12AX7/7025 specs an input C of 1.8p while the 12AY7 is only 1.3p. Not huge but when multiplied times the gain it may be significant. Also the mu of the 12AY7 is a bit less also which reduces Cm.
Of course loading the output of the phono preamp with a CF solves the drive problem pretty much.
Now here is another possibility to throw in the mix. I would like to hear from others who have built a lot of phono stages of different types. It may be possible that good basic performance would be more easily achieved with a feedback based phono stage. The passive version may be more lively but I would think that the NFB in the active type should address a lot of the distortion and drive issues that might otherwise come up and I bet that achieving "perfect" eq curve would be easier.
Just a few thoughts to stir the pot and see what rises to the surface.
Also it seems that the main problems with loading on the 12AX7 would be the effect on the eq circuit that it drives and the distortion that results from less than optimum load. The former can be dealt with by taking the rp into account in the calculation of the eq circuit values (simulation with LTspice could be useful here).
Mitigating the distortion problem is also the fact that the voltage swing will be comparatively small which will limit the effect of non-linearities. By choosing the the values used in the network you may be able to come up with something that though far from ideal might be quite listenable. Of course the mu-follower (or even putting a CF between the AX7 and the eq network) would pretty well solve the distortion issue. It is getting a little more complex but the extra tubes are not necessarily expensive (even 12AU7 can be used for follower or plate load).
Another thought is that the 12AY7 is not super expensive and would provide less of a Cm problem. I looked at the GE data sheets for all three tubes and the 12AX7/7025 specs an input C of 1.8p while the 12AY7 is only 1.3p. Not huge but when multiplied times the gain it may be significant. Also the mu of the 12AY7 is a bit less also which reduces Cm.
Of course loading the output of the phono preamp with a CF solves the drive problem pretty much.
Now here is another possibility to throw in the mix. I would like to hear from others who have built a lot of phono stages of different types. It may be possible that good basic performance would be more easily achieved with a feedback based phono stage. The passive version may be more lively but I would think that the NFB in the active type should address a lot of the distortion and drive issues that might otherwise come up and I bet that achieving "perfect" eq curve would be easier.
Just a few thoughts to stir the pot and see what rises to the surface.
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Active EQ phono preamps are more vulnerable to overload than passive EQ setups are. Also, tubes are not especially good a driving feedback loops. Some active designs use a CF to drive both the loop and the O/P. Again, that's quite a burden to bear.
What could be interesting is a design that passively rolls the highest freqs. off and actively takes care of the other 2 time constants. Such a design will not be low budget, as the OP requested.
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Wow the 12AX7 seems like the spawn of Satan......so unmanageable and difficult...at least on the cheap as I hoped originally.
I can see that this is probably the part of the chain that requires the greatest care in design and execution given the two-headed problem of achieving higher gain but w/ the lowest possible noise.
Hmmmm..where to now for me?
I can see that this is probably the part of the chain that requires the greatest care in design and execution given the two-headed problem of achieving higher gain but w/ the lowest possible noise.
Hmmmm..where to now for me?
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Bingo. "Classic" phono stages... suck. Sorry, there's no better way to put it. Dynamic distortion, drift of RIAA with tube aging, high distortion, poor overload performance, gross slew limiting.
Not what you want to hear, but true nonetheless: if you want a good phono stage on a budget, it will be solid state. If you want a good tube phono preamp, it will cost $$$.
Ok....anyone care to weigh on on this semi-diy kit offering?
At $79.00 less power supply and input/outputs RCAs what do you think?
Marantz 7C Phono » diytubekits.com - Hi Fidelity WITHOUT the Hi Price - DIY Tube Kits
100916114439_m7c_phono_manual_pg_1
I see it still uses the 12AX7/ and/or 12AU7 combination and is based on a "classic" Marantz 7 phono stage.
At $79.00 less power supply and input/outputs RCAs what do you think?
Marantz 7C Phono » diytubekits.com - Hi Fidelity WITHOUT the Hi Price - DIY Tube Kits
100916114439_m7c_phono_manual_pg_1
I see it still uses the 12AX7/ and/or 12AU7 combination and is based on a "classic" Marantz 7 phono stage.
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I can sympathize with your frustration. You can read all these forum postings until you're blue in the face, and you still won't find much agreement on what designs are better than others. Or tubes. Or components. For my part, I will just put in a plug for the Stephen Robinson Izzy Wizzy phono design. His website chronicles the evolution of his design, and I have had the pleasure of building several variations. They have all sounded remarkably good. Here's a link to his website:
Izzy Wizzy Audio
I would recommend starting with one of his basic designs such as the Mk.15 schematic---D3a and 5687; passive eq; battery bias; unregulated power supplies; no interstage transformers. Running the D3a in triode sounds very nice; pentode is even better. You can check some of the later designs for the pentode circuit. You don't have to follow Stephen's power supply circuit very closely. It just needs to be well-filtered with good sounding filter caps and chokes. You CAN spend a lot of money on component parts if you want to. My unit uses a lot of Black Gate caps which are no longer available. But you can also get by with much less expensive parts and still end up with a really good sounding MM phono unit. Of course, for a low-output MC, you just add a good stepup transformer on the input, and you're good to go. Again, you can spend a lot of money but you can also get by with something a lot cheaper. I use Jensen 44-DX transformers which sound quite good. The pair is slightly more than $200. That's not much for opening up the whole world of MC possibilities.
Dave
Izzy Wizzy Audio
I would recommend starting with one of his basic designs such as the Mk.15 schematic---D3a and 5687; passive eq; battery bias; unregulated power supplies; no interstage transformers. Running the D3a in triode sounds very nice; pentode is even better. You can check some of the later designs for the pentode circuit. You don't have to follow Stephen's power supply circuit very closely. It just needs to be well-filtered with good sounding filter caps and chokes. You CAN spend a lot of money on component parts if you want to. My unit uses a lot of Black Gate caps which are no longer available. But you can also get by with much less expensive parts and still end up with a really good sounding MM phono unit. Of course, for a low-output MC, you just add a good stepup transformer on the input, and you're good to go. Again, you can spend a lot of money but you can also get by with something a lot cheaper. I use Jensen 44-DX transformers which sound quite good. The pair is slightly more than $200. That's not much for opening up the whole world of MC possibilities.
Dave
The lengths people will go to to avoid a simple regulator (er, circuit "trick") are shocking to me. Otherwise it's pretty nice.
If you are really set on using a 12AX7, how about a simple JFET source follower before it? Could solve the issue for $8.
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